The operation mode of a power system changes at any time, and the change of the operation mode will cause the fluctuation of a bus voltage. The power system has strict requirements on the fluctuation range of the bus voltage, therefore, a technology for regulating the bus voltage is needed. The most direct manner of regulating the voltage is to change a transformer tap. However, in a load transmission process of the power system, an on-load tap changer with a very high technical content is required for changing the transformer tap without power outage. Reactive on-load tap changers and resistive on-load tap changers are mainly adopted in the world at present. U.S. Pat. No. 3,176,089, U.S. Pat. No. 5,128,605 and U.S. Pat. No. 7,880,341 disclose the reactive on-load tap changers, and U.S. Pat. No. 4,081,741 and U.S. Pat. No. 4,520,246 disclose the resistive on-load tap changers. The reactors of the reactive on-load tap changers are energized for a long term, are relatively large in volumes and are only adopted in the America in the world, and the resistive on-load tap changers are generally adopted in other countries. The resistive on-load tap changers suffer a heating problem, and a significant rise in temperature will be generated by switching the taps of the on-load tap changers for multiple times within a short period of time. Therefore, the switching time of the on-load tap changers within a certain time is strictly limited.
The performance of the on-load tap changer is improved by a thyristor circuit in the U.S. Pat. No. 4,622,513. One of the invention points is that when the switch of a switched current path is switched off, an overvoltage triggering thyristor circuit of a switching path is automatically switched on to quickly splice and switch load current. The defect of the overvoltage triggering thyristor circuit lies in that very large pulse interference is generated every 10 milliseconds. Therefore, adequate anti-interference measures and safety measures are needed to ensure the reliable work of the on-load tap changer. Another invention point of the invention is that, a bidirectional parallel thyristor is triggered by a current transducer to assist a mechanical switch to disconduct the switched current path; the bidirectional parallel thyristor is connected with the mechanical switch in parallel, and the bidirectional parallel thyristor may be switched on by the pulse interference by mistake to cause short circuit circulation. Therefore, the overvoltage triggering thyristor circuit in the invention is serially connected with a transition resistor, in order to limit the possible short circuit circulation to improve the operation safety of the thyristor; thus in U.S. Pat. No. 4,622,513, the heating of the transition resistor is only reduced, while the heating problem of the transition resistor is not solved completely. The U.S. Pat. No. 7,595,614 is an improvement of U.S. Pat. No. 4,622,513. In the U.S. Pat. No. 7,595,614, the transition resistor serially connected with the overvoltage triggering thyristor circuit is cancelled, the heating problem of the transition resistor is solved; since the transition resistor limiting the short circuit circulation is cancelled, in the case of the short circuit circulation, the short circuit circulation is very large; in U.S. Pat. No. 7,595,614, protection is only achieved by a fuse, and the reaction speed of fuse protection is slow, so the safety is poor. In the U.S. Pat. No. 7,595,614, the bidirectional parallel thyristor is still triggered by the current transducer to disconduct the switched current path, and no new anti-interference measure is added, thus the reliability is poor.
In the U.S. Pat. No. 4,622,513 and U.S. Pat. No. 7,595,614, a bidirectional parallel thyristor switching circuit is triggered by the secondary current of a current transducer to switch on and cut off a bidirectional parallel thyristor, and the reliability of a trigger circuit is poor. In the U.S. Pat. No. 4,622,513 and U.S. Pat. No. 7,595,614, a traditional complicated mechanical cam sliding mechanism and an energy accumulating mechanism are stilled adopted, thus the operation vibration and the noise are large; failure is liable to happen, and more frequent operation cannot be implemented.